1. Field of the Invention
The present invention relates to a method of semiconductor manufacturing and more particularly to a method for implantation of carbon-containing clusters into a substrate for stress engineering and diffusion control to improve the characteristics of
2. Description of the Prior Art
The Ion Implantation Process
The fabrication of semiconductor devices involves, in part, the formation of transistor structures within a silicon substrate by ion implantation. As disclosed by Sferlazzo et. al. in U.S. Pat. No. 5,497,006, ion implantation equipment includes an ion source which creates a stream of ions containing a desired dopant species, a beam line which extracts and accelerates the ions from the ion source by means of an extraction electrode, and forms and focuses the ion stream into an ion beam having a well-defined energy or velocity, an ion filtration system which selects the ion of interest, since there may be different species of ions present within the ion beam, and a process chamber which houses the silicon substrate upon which the ion beam impinges; the ion beam penetrating a well-defined distance into the substrate. Transistor structures are created by passing the ion beam through a mask formed directly on the substrate surface, the mask being configured so that only discrete portions of the substrate are exposed to the ion beam. Where dopant ions penetrate into the silicon substrate, the substrate's electrical characteristics are locally modified, creating source, drain and gate structures by the introduction of electrical carriers: such as, holes by p-type dopants, such as boron or indium, and electrons by n-type dopants, such as phosphorus or arsenic, for example.
A recent development in semiconductor processing is the incorporation of mechanical stress to enhance transistor performance. This stress is generated by including atoms of elements other than silicon into a lattice structure. The successful process to date has been the incorporation of Ge atoms into the source and drain regions of a PMOS transistor. Inclusion of Ge atoms into a silicon substrate forms a SiGe alloy, which has a compatible lattice structure with the Si lattice. However, the Ge atoms are larger than the Si atoms, resulting in a larger lattice constant for the SiGe alloy, which can be controlled by the amount of Ge included. By forming this alloy material in the source and drain region of a PMOS transistor, the larger lattice therein places the channel region under compressive stress, which enhances the hole mobility and increases the performance of the PMOS transistor. The inclusion of Ge atoms only works for PMOS transistors because compressive stress is detrimental to the electron mobility and degrades the performance of an NMOS transistor.
Prior Art Ion Sources
Traditionally, Bernas-type ion sources have been used in ion implantation equipment. Such ion sources are known to break down dopant-bearing feed gases, such as BF3, AsH3 or PH3, for example, into their atomic or monomer constituents, producing the following ions in copious amounts: B+, As+ and P+. Bernas-type ion sources are known as hot plasma or arc discharge sources and typically incorporate an electron emitter, either a naked filament cathode or an indirectly-heated cathode. This type of source generates a plasma that is confined by a magnetic field. Recently, cluster implantation ion sources have been introduced into the equipment market. These ion sources are unlike the Bernas-style sources in that they have been designed to produce “clusters”, or conglomerates of dopant atoms in molecular form, e.g., ions of the form Asn+, Pn+, or BnHm+, where n and m are integers, and 2≦n≦18. Such ionized clusters can be implanted much closer to the surface of the silicon substrate and at higher doses relative to their monomer (n=1) counterparts, and are therefore of great interest for forming ultra-shallow p-n transistor junctions, for example in transistor devices of the 65 nm, 45 nm, or 32 nm generations. These cluster sources preserve the parent molecules of the feed gases and vapors introduced into the ion source. The most successful of these have used electron-impact ionization, and do not produce dense plasmas, but rather generate low ion densities at least 100 times smaller than produced by conventional Bernas sources. For example, the method of cluster implantation and cluster ion sources has been described by Horsky et al. in U.S. Pat. No. 6,452,338 and U.S. Pat. No. 6,686,595 hereby incorporated by reference. The use of B18H22 as an implant material for ion implantation of B18Hx+ in making PMOS devices is disclosed in Horsky et al. in pending U.S. patent application Ser. No. 10/251,491, published as U.S. Patent Application Publication No. US 2004/0002202 A1, hereby incorporated by reference.